Therapeutic nucleic acids include, e.g., small interfering RNA (siRNA), microRNA (miRNA), antisense oligonucleotides, ribozymes, plasmids, and immune-stimulating nucleic acids. These nucleic acids act via a variety of mechanisms. In the case of interfering RNA molecules such as siRNA and miRNA, these nucleic acids can down-regulate intracellular levels of specific proteins through a process termed RNA interference (RNAi). Following introduction of interfering RNA into the cell cytoplasm, these double-stranded RNA constructs can bind to a protein termed RISC. The sense strand of the interfering RNA is displaced from the RISC complex, providing a template within RISC that can recognize and bind mRNA with a complementary sequence to that of the bound interfering RNA. Having bound the complementary mRNA, the RISC complex cleaves the mRNA and releases the cleaved strands. RNAi can provide down-regulation of specific proteins by targeting specific destruction of the corresponding mRNA that encodes for protein synthesis.
The therapeutic applications of RNAi are extremely broad, since interfering RNA constructs can be synthesized with any nucleotide sequence directed against a target protein. To date, siRNA constructs have shown the ability to specifically down-regulate target proteins in both in vitro and in vivo models. In addition, siRNA constructs are currently being evaluated in clinical studies.
However, two problems currently faced by interfering RNA constructs are, first, their susceptibility to nuclease digestion in plasma and, second, their limited ability to gain access to the intracellular compartment where they can bind RISC when administered systemically as free interfering RNA molecules. These double-stranded constructs can be stabilized by the incorporation of chemically modified nucleotide linkers within the molecule, e.g., phosphothioate groups. However, such chemically modified linkers provide only limited protection from nuclease digestion and may decrease the activity of the construct. Intracellular delivery of interfering RNA can be facilitated by the use of carrier systems such as polymers, cationic liposomes, or by the covalent attachment of a cholesterol moiety to the molecule. However, improved delivery systems are required to increase the potency of interfering RNA molecules such as siRNA and miRNA and to reduce or eliminate the requirement for chemically modified nucleotide linkers.
In addition, problems remain with the limited ability of therapeutic nucleic acids such as interfering RNA to cross cellular membranes (see, Vlassov et al., Biochim. Biophys. Acta, 1197:95-1082 (1994)) and in the problems associated with systemic toxicity, such as complement-mediated anaphylaxis, altered coagulatory properties, and cytopenia (Galbraith et al., Antisense Nucl. Acid Drug Des., 4:201-206 (1994)).
To attempt to improve efficacy, investigators have also employed lipid-based carrier systems to deliver chemically modified or unmodified therapeutic nucleic acids. Zelphati et al. (J. Contr. Rel., 41:99-119 (1996)) describes the use of anionic (conventional) liposomes, pH sensitive liposomes, immunoliposomes, fusogenic liposomes, and cationic lipid/antisense aggregates. Similarly, siRNA has been administered systemically in cationic liposomes, and these nucleic acid-lipid particles have been reported to provide improved down-regulation of target proteins in mammals including non-human primates (Zimmermann et al., Nature, 441: 111-114 (2006)).
In spite of this progress, there remains a need in the art for improved lipid-therapeutic nucleic acid compositions that are suitable for general therapeutic use. Preferably, these compositions would encapsulate nucleic acids with high efficiency, have high drug:lipid ratios, protect the encapsulated nucleic acid from degradation and clearance in serum, be suitable for systemic delivery, and provide intracellular delivery of the encapsulated nucleic acid. In addition, these nucleic acid-lipid particles should be well-tolerated and provide an adequate therapeutic index, such that patient treatment at an effective dose of the nucleic acid is not associated with significant toxicity and/or risk to the patient. The present invention provides such compositions, methods of making the compositions, and methods of using the compositions to introduce nucleic acids into cells, including for the treatment of diseases.